Helicobacter pylori is a helical rod shaped bacterium that colonizes the human stomach causing clinical outcomes that range from mild gastritis to peptic ulcer and gastric cancers. Most descriptions of H. pylori virulence include the widely-accepted hypothesis that its helical cell shape enhances colonization of the stomach. We recently provided the first experimental support for this theory with the characterization of four cell shape determining genes (csd1, csd2, csd3, ccmA) that promote helical cell curvature and twist through changes in cell wall peptidoglycan (PG) crosslinking and are required for efficient stomach colonization. Alteration of peptide crosslinking within the PG sacculus defines a new mechanism for bacterial cell shape generation; previous studies delineated localized deposition and restriction of new PG deposition as drivers of straight rod and curved rod shapes. Though the mutants described above have lost helical twist, they retain some curvature. Thus much remains to be learned about the precise molecular mechanisms for generating helical shape in H. pylori. This grant builds on our published work with a goal of elucidating the H. pylori helical shape generating program and the contributions of this cell shape program to H. pylori pathogenesis. Aim 1 will identify and characterize additional genes involved in helical cell shape determination. Aim 2 will investigate how these genes work together to promote shape using biochemical studies of cell shape proteins and PG cell wall composition. Aim 3 will investigate possible mechanisms by which the helical cell shape generating program promotes stomach colonization including modulation of swimming behavior and niche utilization within the stomach. Bacteria manifest an impressive diversity of cell shapes that are highly conserved within species but the selective forces leading to conservation of specific shapes are poorly understood. We have established H. pylori as an excellent model to elucidate molecular determinants of helical cell shape and the selective role of shape during host colonization in the Proteobacteria; Csd proteins and CcmA homologues are well conserved among curved to helical Proteobacteria including several other pathogens such as Campylobacter jejuni and Vibrio cholerae. The elucidation of a helical shape generating program required for stomach colonization promises to illuminate new targets for antimicrobial design which are badly needed in H. pylori as current strains display increasing resistance to existing therapies and fits the mission of NIAID to understand and treat infectious diseases.